Setting a motor to the rotation axis of a turbocharger is known in conventional art. Such a motor-driven turbocharger is driven to assist the rotation of the compressor. It is therefore possible to attain a necessary level of boost pressure when necessary regardless of whether the exhaust energy supplied from the internals combustion engine is large or small. Thus, the motor-driven turbocharger can realize high boost pressure by forcibly driving the compressor even when the engine rotation speed is low, that is, when the exhaust gas flow to drive the turbine is low. However, the motor-driven turbocharger has a problem that the likelihood of exceeding the compressor's surge limit is high.
As a method to avoid surge, it is effective to secure a certain amount of air which passes the compressor. An internal combustion engine having a motor-driven supercharger, disclosed in Japanese Patent Laid-Open No. 2004-332715, is provided with a bypass passage between the upstream and downstream of the compressor to bypass the compressor and a bypass valve which opens and closes this bypass passage. According to this technique, the amount of air which passes the compressor can be increased to avoid surge. For this purpose, the air boosted by the compressor is partly returned to the upstream of the compressor by opening the bypass valve (hereinafter denoted as “air-bypass”).